1. Field of the Invention
This invention relates to novel gypsum compositions, a process for preparing these materials and articles containing the materials. More specifically, the materials include as an additive a latex polymer and a surfactant. This combination surprisingly provides strength to the gypsum composition so that lighter weight materials, particularly wallboard can be produced having less weight.
2. Technology Description
Gypsum board is used to make residential and commercial building interior walls. It has advantages over plaster walls because it is relatively easy to install, has lower costs, and requires minimal finishing. Gypsum board is normally supplied in sheets varying from 0.25 to 1 inch in thickness, 48 inches wide and in lengths between 8 and 16 ft. Fifty-four inch wide boards are made for buildings with 9-foot ceilings. The majority of gypsum board made is 4'.times.8'1/2" thick.
Gypsum board is relatively dense. When shipped, trucks are limited by weight rather than volume. Since a significant portion of the cost of gypsum board is freight, it is desirable to reduce the weight of gypsum board without sacrificing strength. In addition, lighter boards are easier to handle and install.
Lighter weight structural units can be made in larger sizes, if desired, which can also reduce manufacturing costs per unit of area.
In order to be of any commercial value, the gypsum boards must be able to pass the objective testing criteria of ASTM Methods C-36 and C-473.
Commercial gypsum board contains about 70% air by volume. About 30% of the volume is due to the incorporation of air bubbles. As the volume of air bubbles increases, the strength of the board dramatically decreases. Experience has taught that entrainment of air alone will not product a sufficiently improved lightweight wallboard that has adequate strength. Further, the incorporation of lightweight particles, alone, even at a low levels, does not produce a satisfactory lightweight product.
Problems that have been encountered, lie in the area of dispersing the lightweight articles thoroughly throughout the wet cement or calcined gypsum mixture, and firmly adhering such particles in the cured cementitious matrix. Originally, technologies for decreasing the weight of structural units involved the use of expanded vermiculite, perlite and the like. In a more recent patent, lightweight particles or beads of expandable thermoplastic polymeric resins have been used. They have been primarily expandable polystyrene beads, as well as polyethylene and various polymeric copolymers, which are generally expanded before use to achieve the lowest density. These thermoplastic particles or beads are advantageous in that each particle or bead comprises closed cells and when they are pre-expanded may be readily made to as low a density as 1 pound per cubic foot or even as low as about 0.5 or 0.6 pounds/cubic foot. Generally, preexpansion on a commercial basis will provide particles from as low as about 0.6 pounds per cubic foot to about 11/2 pounds per cubic foot without extra and costly equipment and additional processing. Thus, the thermoplastic lightweight particles are a great deal more advantageous than those previously used of vermiculite and/or perlite which have a density at least several times that of the thermoplastic granules.
Despite the improvements associated with the incorporation of particles, brittleness of the cementitious material may result. In a lightweight wallboard, the volume occupied by the thermoplastic and/or entrained air materially reduces the amount of gypsum matrix present and lowers strength and especially the desired flexural strength.
Another major problem that has been encountered in the high speed commercial production of lightweight structural units has been the removal of excess water from the slurry or mixture during the curing. A certain amount of water is required to hydrate the dry cementitious material such as calcined gypsum or cement. To obtain a free flowing mixture, it has been necessary to add excess water on the order of two or three or more times that actually needed for hydration to provide a smooth, free-flowing, low viscosity mixture suitable for use in transporting and placing the mix into a mold or other means where the slurry or mix is dried to form the core for the lightweight structure. The energy, time, equipment and space required for the removal of the excess moisture create a critical obstacle in the low cost manufacture of lightweight, high speed, high volume production of structural units, such as gypsum wallboard and the like.
U.S. Pat. No. 4,265,964 provides a method for producing lightweight gypsum in which low density expandable thermoplastic granules or particles that have been expanded are used in conjunction with a cementitious base material, such as, gypsum (generally calcined to hemihydrate form, i.e. CaSO.sub.4 . 1/2H.sub.2 O); a surfactant; an additive which acts as a frothing agent to incorporate an appropriate amount of air into the mixture to aid in obtaining a light density; a film forming component to provide better flow and consistency of the mixture, as well as greater strength, flexibility, water resistance and adherence of the final product when the mixture is cured; and a starch which may be added as a binding agent and to increase the adhesive properties of the mixture. This patent requires the presence of the particulate material, which increases the overall cost of the final product.
It has been suggested in the literature that improvements in the final gypsum product can be obtained if one to 50 percent by weight of the final product of latex is added. See, for example, U.S. Pat. No. 5,401,798, JP63307175 A2(CA 111:44452), JP85018536 B4(CA 103:88836), JP 60021875 A2(CA 103:10530), JP 57038360 A2 (CA 97:43238), U.S. Pat. No. 4,137,198, JP 50041943 (CA 84:78875). This proposed solution is deficient in that it is imperative that the latex be dispersed throughout the gypsum product, or else potential areas of weakness can occur in the board. While the use of dispersants or surfactants could be utilized to assist in the dispersion of the latex, one skilled in the art would inevitably be led to the conclusion that minimal amounts of dispersants or surfactants be added in order to preserve board strength. Furthermore, the addition of many types of surfactants would lead one skilled in the art to conclude that the viscosity of the slurry used to form the final product would increase to a level which is undesirable for commercial manufacture.
Despite the above teachings, there still exists a need in the art for a gypsum product which, when produced into wallboard, satisfies the criteria of ASTM Methods C-36 and C-473, needs no added particulate material, and, despite having a decrease in weight per unit volume, maintains or demonstrates an increase in strength as compared to the higher density products. The present invention is directed to meeting such objectives.